The ABI have already said that the government hasn't discussed this with them yet and can't see how it'll produce lower premiums, given that less than 0.1% of motor claims involve cyclists.
But how many % of motor claims involve drivers who are also trained cyclists compared to drivers who aren't? If it's dispropotionate, then that might suggest lower premiums would be justified.
One possibility suggested on another site is that the government might be looking to give some discount or rebate on the insurance premium tax to cyclists.
I'd expect take-up to be at least as good as PassPlus's (£180 course for a short-term reduction at best) - but I can't find anything say how many people do PassPlus each year.
Government pushing this rather than the insurers makes me doubt it will happen.